Anthony Zerbe?

I think this was not only Gene Hackmans and John Davis Chandlers first movie, but possibly also Anthony Zerbe's debut.

I'm almost positive, he is one of the guys unloading the 'Ventura' (the one in front on the left), visibly in two shots before and after Coll opens fire on them.

The guy at least has Zerbe's haircut und seems to move like Zerbe, as I remember him from other roles and similar getting-killed-scenes.
As Anthony Zerbe received his first credit in 1963, this actually could be possible.